The present invention relates to a composite sheet used for reproducible electrostatic image display or record by a direct electrification method. Such a composite sheet is used in apparatus for displaying, transmitting and recording a large sized image information such as large displays, large facsimile machines and large screen copying machines that utilize electrostatic latent images.
Method of displaying, transmitting and recording electrostatic images by utilizing electrostatic latent image is generally classified into two groups, i.e., indirect electrification (charging) system and direct electrification (charging) system. As the material for electrostatic recording used in the indirect charging system, photoconductive material prepared by laminating a photo-semiconductor film such as a selenium-evaporated film, poly-N-vinylcarbazole or a charge transfer complex of poly-N-vinylcarbazole and trinitrofluorenone on an electroconductive layer has been put to practical use as the material for constituting a photo-conductive drum in a copying machine. In addition, electrostatic recording paper prepared by coating a material obtained by dispersing photo-conductive material such as zinc oxide, lead oxide or titanium oxide in a highresistance adhesive resin, on an electrified substrate paper has also been used generally as recording paper for copying machine or facsimile machine. However, among the composite materials for the electrostatic recording used in the indirect charging system, those using a photo-semiconductor film such as a selenium-evaporated film can be used reproducibly but, since it is difficult to obtain a film of large area and its production cost is expensive, it is attended with a remarkable economical difficulty in producing an electrostatic recording machine such as a large-sized copying machine or facsimile machine by using this material. On the other hand, electrostatic recording paper prepared by coating a resin containing photoconductive material such as zinc oxide on an electroconductive paper can easily be produced into a recording paper of large area and the production cost thereof is inexpensive. However, since such a material is used paper as a substrate, it has a drawback that electroconductivity changes along with the humidity change, and the strength and durability are poor. Furthermore, since it can not be used repeatingly by erasing once recorded image, the material can not be utilized for reproducible electrostatic image display or record.
Electrostatic recording material based on the direct charging system usually comprises a dielectric layer possessing static electric charges and a semiconductor layer having a function as an electrode, in which image recording is usually conducted by a method of applying directly a voltage to the material, thereby forming electrostatic images on the surface and then visualizing them with toner or the like.
For the electrostatic recording material used in such direct charging system, electrostatic recording paper prepared by laminating a dielectric layer composed of a dielectric resin such as acrylic acid ester resin, acrylic styrene resin, methacrylic resin, vinyl chloride/vinyl acetate copolymer, silicon resin, alkyd resin or epoxy resin and inorganic powders such as barium sulfate, titanium oxide, calcium carbonate or kaolinite on the surface of an electrified substrate paper has been used as the recording paper for copying machines, etc. However, this electrostatic recording paper also has problems in view of durability, humidity proofness, stability of electroconductivity, etc. as the electrostatic recording paper in the indirect charging system previously described above prepared by coating the photo-conductive material, and therefore, it can not be used as the material for reproducible electrostatic image display or record.
As has been described above, since electrostatic recording can easily provide those functions such as copy and transmission collectively as compared with other electronic recording methods, it has an advantageous effect to be applicable to various application uses such as copying machine, facsimile machine, etc. However, since most of conventional electrostatic recording materials can not be used for inexpensive and reproducible large size image display or record, their use is limited only to non-reproducible electrostatic recording paper and the application to large sized display, large sized copying machine or large sized facsimile machine has been extremely difficult.
For instance, in Japanese Patent Application Laid-Open (KOKAI) 64-966 (1989) [corresponds to GB 2206842], a display material comprising a dielectric film of polyvinylidene fluoride, an electroconductive film containing zinc oxide, titanium oxide and titanium as an electroconductive substance and a reinforcing sheet of vinyl chloride is proposed as an electroconductive composite sheet used for an electron black board.
Since the display material disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 64-966 (1989) has a problem that the whiteness is not sufficient and the whiteness is reduced with the change in the passage of time when exposed to light, and the non-mobile image or still image forming performance is reduced with the change in the passage of time in accordance with the increase of the resistance of the electroconductive composite sheet by the repeating use, if a large sized display is manufactured by using such material as the display surface for electrostatic images, it brings about a problem that the sharpness or appearance of the image is deteriorated and it is not quite satisfactory.
In view of the above, an object of the present invention is to provide, with an economical advantage, a material for reproducible large area electrostatic image display that can be used also for apparatus of displaying, transmitting and recording a large area image information, such as large sized displays, large sized facsimile machines and large sized copying machines, while taking the advantage of the electrostatic recording, that is, high speed recording, recording storability and wide application use.
Another object of the present invention is to provide a composite sheet having improved whiteness and light fastness for reproducible electrostatic image display or record capable of obtaining images with sharp contrast when it is used as the display surface of a large size display.
As a result of the present inventors' earnest study for attaining the above-mentioned objects, it has been found that a composite sheet obtained by laminating a highly dielectric film having a specific dielectric constant of not less than 5 on a semiconductor film containing an electroconductive filler, for example, a fibrous or flaky electroconductive filler and having a surface resistivity of from 10.sup.3 -10.sup.10 .OMEGA./.quadrature. is useful as a reproducible electrostatic image display or record material, has a stable non-mobile image or still image forming performance in a circumstance at low to high humidity condition, in which the non-mobile image or still image forming performance is not reduced by the repeating use, and is excellent in the durability. The present invention has been accomplished based on this findings.